1. Field of the Invention
The present invention relates to new optically active oxazoline-carboxylic acid derivatives, specifically optically active trans-2-oxazoline-4-carboxylate derivatives, which can be used in the preparation of optically active amino acids such as threonine, dihydroxyphenylserine and the like, and to a method for preparing the said derivatives.
2. Description of the Prior Art
There are many optically active .beta.-hydroxy-.alpha.-amino acids having important physiological activity. For example, the L-form of threonine, having the threo configuration and having the following formula: ##STR2## is an essential amino acid, which cannot be formed in vivo in animals by biosynthesis.
In addition, an L-threo-3-(3,4-dihydroxyphenyl) serine of the following formula: ##STR3## is known to be useful as a remedy for peripheral orthostatic hypotension (Japanese Patent Application OPI No. 104815/81), a remedy for Parkinson's disease (Japanese Patent Application OPI No. 52219/83) or a diuretic (Japanese Patent Application OPI No. 85318/86). (The term "OPI" is used herein means an unexamined and published application.)
When the above optically active .beta.-hydroxy-.alpha.-amino acids have a substituent in the .beta. position, there are two possible configurations, the threo-form and the erythro-form, and each of these forms include the respective optical isomers. Accordingly, for example, when an L-threo form is to be obtained, it must be obtained from a mixture comprising at least four kinds of stereoisomers. In a previous purification scheme, the erythro form is first removed from the mixture and then the L-threo form is obtained by optical resolution. In the case where the D-form is required, it is prepared in the same manner. This conventional means is, however, defective in that the operation is complicated and the yield is poor.
On the other hand the reaction of an aldehyde and an isocyano-carboxylate has been known to proceed in the presence of an amine such as triethylamine, as follows: ##STR4## Further, compound II above is known to cyclize in the presence of a metal to give a 2-oxazoline-4-carboxylate derivative as follows: ##STR5##
In the above formula the selectivity of forming the D- or L-form in the *1- and *2-positions in the 2- oxazoline-4-carboxylate derivative has not yet been reported.
The above-described conventional chemical synthesis of optically active threo-.beta.-hydroxy-.alpha.-amino acids requires complicated operations which include the separation of the products on the basis of a difference in configuration, followed by successive separation of the optically active isomers. This is because the direct asymmetric synthesis of the desired chemical substance is impossible.
In view of the prior art methods described above, it can be seen that a need continues to exist for a method for preparing a desired optically active amino acid such as an optically active .beta.-hydroxy-.alpha.-amino acid, which is both simple and which results in a higher yield as compared to the known conventional methods. There is also a need to provide new starting materials for the preparation of desired final products, such as optically active amino acids.